Frequency multiplier



Dec. 14, 1965 D,R.LUDW1G ETAL mm FREQUENCY MULTIPLIER Filed Nov. 25. 1960 OSCILLATOR FILTER INVENTORS. DAV/D R. LUDWIG RICHARD RHEWSON TTORNE Y United States Patent C) 3,223,918 FREQUENCY MULTlPLlER @avid R. Ludwig and Richard R. Hewson, Randolph, Mass., assignors to General Electronic Laboratories, liuc., Cambridge, Mass., a corporation of Massachusetts Filed Nov. 25, 1960, Ser. No. 71,823 Claims. (Cl. Zim-69) This invention relates to microwave frequency multipliers and more particularly to high Q parametric microwave frequency multipliers. The term parametric as used herein means a substantially lossless element having `a nonlinear response characteristic such as a capacitor whose capacitance is a function of voltage or an inductor whose inductance is a function of current.

Heretofore the Klystron has been in general the apparatus used for providing radio frequency energy throughout the microwave frequency band and in the lower microwave range planartriodes have sometimes been used. Their relatively high cost, limited life due to such factors as filament burnout, cathode deterioration, relative susceptibility to failure from shock and vibration, relatively low operating efficiency and instability of output frequency presented substantial problems in their use in many applications, particularly where high reliability and extremely accurate operating frequencies are required. Pursuant to the present invention, these problems have been overcome in a parametric microwave frequency multiplier which also embodies other desirable features and advantages. Among these other features and advantages are a parametric microwave frequency multiplier characterized by extreme ruggedness such that it is capable of withstanding high impact and shock conditions as well as high operating temperature conditions and is impervious to wide variations in climatic conditions. A further advantage is that it is not subject to deterioration with time. Another advantage is that it does not require a filament, cathode or D.C. voltage for its operation. And a further advantage of the present frequency multiplier is that it embodies the stability of crystal controlled operation. And a further advantage is that it has an extremely long and in-fact indefinite operating life free of the need for periodic servicing and is substantially impervious to physical breakage or damage.

Also, its heat generation is negligible thereby avoiding the need for auxiliary structures for the dissipation of heat as is required in most other frequency-generating devices in the microwave band.

A primary object of the present invention is the provision of a parametric microwave frequency multiplier which is highly stable in its operation and provides a very accurate source of microwave frequency energy.

Another object of the present invention is the provision of a parametric microwave frequency multiplier which is extremely rugged in its operation.

And a further object of the present invention is the provision of a parametric microwave frequency multiplier having a structure which readily lends itself for crystal controlled operation.

And a still further object is the provision of a parametric microwave frequency-multiplier having a structure which readily lends itself for operation with either a wave guide or coaxial output.

And another object is the provision of a parametric microwave frequency multiplier which may readily be varied in its operating power output.

And a further object is the provision of a parametric microwave frequency multiplier which is readily adaptable for selected frequencies throughout the entire -band of microwave frequencies. And a still further object is the 3,223,9l8 Patented Dec. 14, 1965 provision of a parametric microwave frequency multiplier which is highly efficient in its operation.

These objects, features and advantages are achieved generally by providing an output waveguide cavity filter resonant at the desired output frequency, an input structure adaptable for receiving radio-frequency energy which is a subharmonic of the output frequency, a nonlinear reactance type element in responsive relation to the input frequency for generating harmonics of the input frequency, and a filter structure arranged for transmitting the desired harmonic frequency to the cavity.

By making the nonlinear reactance element in the form of a pill varactor mounted in a rectangular wave guide whose dimension parallel to the electric field has been compressed, high efficiency and stability of operation is thereby achieved.

By making the broad dimension of the Wave guide such that the cut olf frequency of the wave guide lies above the harmonic of the input frequency which is nearest and below the output frequency, harmonics lower than the desired harmonic will not appear in the output.

By making the cavity at the output tuned to the desired output frequency and a tapered transition from the varactor to the cavity maximum transmission of the desired and maximum rejection of the undesired harmonics are thereby achieved.

By placing the cavity a distance from the varactor such that a voltage minimum is reflected at the plane of the varactor for the nearest higher undesired harmonic above the output frequency, the best reactive termination of the varactor at this frequency is thereby achieved. Higher harmonics can generally be neglected because of the rapid decline of the conversion efficiency to higher harmonics.

By providing a backshort with distance from the varactor such as to reflect a voltage minimum across the varactor for undesired harmonics in the vicinity of the desired output frequency above the cut-off frequency of the waveguide, thereby with the resonant cavity placement achieves voltage minimum at the varactor for all important undesired harmonics above the cut-off frequency of the waveguide.

By making the vinput structure in the form of a coaxial line coupled from a low-pass filter to the varactor and having an electrical length such that a voltage minimum is reflected from the low-pass filter across the coaxial line at its junction with the inside face of the waveguide for harmonics of the input frequency below the output frequency, the best termination to the varactor at these frequencies is thereby achieved. This condition together with the backshort and cavity placement permits necessary idler currents to flow through the varactor for undesired harmonics.

`By providing a rectangular filter configuration whose width is determined by the width of the wave guide for the desired frequency of operation with a dimension such as to be at cut-off for the next lower harmonic to the desired output frequency and whose length is substantially one-half the wave length of the resonant frequency simplicity of structure and improved operation is thereby achieved.

By providing iris structures dening the boundaries of the length of the resonant cavity, a waveguide configuration of the filter structure for applying the desired harmonic to the cavity may thereby be used.

By making the waveguide filter structure in the form of a rectangular waveguide with the side parallel to the electric field being dimensionally compressed in the form sometimes referred to as a toll ticket for carrying the reactive element with a waveguide transition from the low impedance of the toll ticket to the normal impedance of the waveguide at the iris, a rugged and relatively simple unitary filter structure for the filter and resonant cavity is thereby achieved.

By providing an adjustable capacitative probe at the signal output side of the toll ticket, adjustment of the desired signal frequency power reaching the resonant cavity is thereby achieved.

By making the input structure in the form of a coaxial cable coupled from a radio frequency filter to the reactive element and making the effective electrical length of the coaxial cable such that, when taken with the filter, a voltage minimum is refiected at the reactive element for all undesired significant harmonics of the input frequency signal from the source of radio frequency energy, the elimination of the need for matching the impedance of the frequency source to the input of the reactive element is thereby achieved. By providing an adjustable capacitative tuning probe 1n the electric field plane of the resonant cavity convenient adjustment of the cavity for the desired frequency of resonance is thereby achieved and particularly to adjust for the affect of the iris on cavity resonance.

By replacing the iris in the output side of the resonant cavity by a looping coupling, operation with coaxial cable output is thereby achieved.

By providing at the input structure a coaxial cable connector arrangement for supporting the reactive element a fiexible coaxial cable to the signal frequency source filter may thereby be used.

These features, objects and advantages, will be more clearly understood from the following description taken in connection with the accompanying drawings of preferred embodiments of the invention and wherein:

FIG. 1 is a partly isometric and a partly block diagram of a preferred embodiment of a parametric microwave frequency multiplier made in accordance with the present invention and coupled to a filter and radio frequency signal source.

FIG. 2 is a cross section of the frequency multiplier shown in FIG. 1 together with the input frequency source filter shown in block form.

FIG. 3 is a section on line 3-3 of FIG. 2.

FIG. 4 is a section on line 4-4 of FIG. 2.

FIG. 5 is a partially cutaway section showing an alternative coaxial cable arrangement for the frequency multiplier shown in FIG. l.

FIG. 6 is a cutaway sectional view of a portion of the frequency multiplier shown in FIG. 2 arranged for operation with a coaxial line output.

Referring to FIG. 1 in more detail, -a parametric microwave frequency multiplier made in accordance with the present invention is designated generally by the numeral 10. The frequency multiplier 10 has a resonant cavity 12 having a rectangular cross sectional configuration wherein the width 14 and height 16 may be the width and height respectively of a conventional waveguide configuration for a desired operating frequency. The resonant cavity 12 has irises 18 and 20 defining the cavity length 22 which is equal to substantially one-half the wavelength of the desired output frequency signal. The orifice 26 in the iris 20 is proportioned for maximum energy coupling to the cavity 12. The orfce 24 in the iris 18 is proportioned for maximum output of radio frequency energy from the cavity 12 and the proper impedance match reflected from the load (not shown) to the cavity 12.

An adjustable capacitive probe 28 Which may be in the form of a body of a machine screw is provided in the E-Plane of the cavity 12 for tuning to a desired resonant frequency of operation. The input side of the cavity 12, because of its retcangular cross sectional construction as described, may thereby be an integral part of a normal waveguide 30 which with a waveguide transition 32 and a low impedance section 34 having its height 33 compressed or shortened as compared to the normal waveguide height 16 and often referred to as a toll ticket form a configuration important to the transmission of maximum energy at the desired signal frequency to the resonant cavity 12.

The height 33 has been shortened to reduce the impedance of the waveguide at the toll ticket 34 to match the impedance of a varactor 44 housed by the toll ticket 34. The proper height 33 may be emperically found for a particular output frequency and frequency multiplication factor by measuring the power output at the desired output frequency for several heights 33 and graphically plotting for the height 33 at which the output power is at a maximum.

A boss or lug 36 fixed in electrical engagement with the lower surface of the low impedance toll ticket 34 is arranged to rigidly hold an electrical conductive ground contact element 38 as by a set screw 40. The ground contact element 38 has a shallow recess 42 at its upper end supporting one terminal of a varactor 44 and providing electrical grounding engagement therewith.

The other terminal of the varcator 44 is firmly held in electrical -engagement by a similar recess 46 in the inner electrical conductor 48 of a coaxial line 50 having an outer conductor 52 coupling the toll ticket 34 to a radio frequency signal source filter 54. The signal source filter 54 may be of conventional design having a configuration for preventing undesired harmonics generated from a frequency source such as an oscillator 56 from exciting the varcator 44 and for preventing harmonics generated in the varcator from reaching the oscillator 56. The signal source oscillator 56 generates signals at a frequency which is a subhar-monic such as 1/z or 1/10 of the frequency of the cavity 12.

It has been found that the greatest effectiveness of this corrective filtering arrangement is achieved by selecting the coaxial line 50 with an electrical length 58, such that when taken with the effective electrical length for selected harmonic frequencies in the filter 54, a voltage minimum is reflected at the varcator 44 for the undesired harmonic frequencies below the selected output frequency. By way of example and not limitation Iherein the source filter 54 may be of conventional design such as illustrated on page 668 of the publication entitled, Very High Frequency Techniques, vol 2 published 'by McGraw-Hill Book Co., Copyright 1947.

Any suitable oscillator 56 may be used depending upon such factors as output frequency and accuracy of output frequency desired. The present invention lends itself particularly well to extremely accurately maintained output frequencies by the use of crystal controlled oscillators for the oscillator 56 such as, for example, shown on page 496, FIG. 15, of the publication entitled Radio Engineers Handbook, 1945 edition, Mc- Graw-Hill Book Co.

The length of the coaxial transmission line 60 coupling the oscillator 56 and filter 54 is not normally critical unless there is mismatch between the output of the oscillator and the impedance of the line 60 and a mismatch between the coaxial line 60 and the input of the filter 54 is producing unwanted harmonics, in which event the length of the coaxial line 60 is proportioned as a transformer between the impedance of the oscillator 56 and the impedance at the input terminals of the filter 54. Alternatively, in place of such transformer structure 60, either a convention coaxial line tuner or a coaxial attenuator may be used for the impedance transformation.

While the coaxial line S0 is shown as having a rigid construction, it alternatively may be arranged with fiexible coaxial cable 62 by providing a connector 64, such as that shown in FIG. 5, at both the filter 54 and the toll ticket 34. In the present instance the connector 64 is of the type N variety with a support insert 66 of dielectric material as Tefion for holding the inner conductor 48 in place. However, it should be understood that the connector 64 is shown as a type N connector for illustrative purposes only and not for purposes of limitation, in that other types of coaxial cable connectors 'may also lbe used in accordance with good coaxial conductor practice.

A standard type coupling member flange 68 has been provided at the output of the cavity 12 for coupling to a desired waveguide load (not shown). Alternatively the output of thc cavity 12 may be coupled through a coaxial cable as shown in FIG. 6 wherein a loop coupling 70 to the inner conductor 69 of a coaxial line 71 is provided in conventional manner. While loop coupling has been shown in FIG. 6, it is to be understood that probe coupling and other conventional coupling arrangements may also be Iused.

In the operation of the present invention, the signal source oscillator 56 provides a fundamental signal frequency which is a submultiple of the desired output signal frequency through the coaxial cable 60, the filter 54 and coaxial line 50 to the varactor 44 in the low impedance toll ticket 34. The length 53 of the coaxial line S0 proportioned as described above, effectively cooperates with filter 54 to prevent harmonics of the fundamental signal'frequency from the varactor, from reaching the oscillator 56 and confines excitation frequency to the varactor to that of the fundamental frequency of the oscillator 56.

Even though only the fundamental frequency from the oscillator 56 is thereby permitted to excite the varactor 44, its non linear reactive structural characteristic is such that this fundamental frequency would tend to excite it to cause the generation of a multiplicity of harmonics of the fundamental frequency. However, a closure 72 provides a back cavity in the toll ticket 34 and having a length 74 to the center of the varactor 44 proportioned to reflect a voltage minimum across the varactor 44 for undesired harmonic frequencies of the fundamental above the cut-off frequency of the waveguide and in the vicinity of the desired output frequency. The length 74 may be found empirically or by calculation and may theoretically be such as to prevent the generation of all unwanted harmonics. However, as a practical matter only the most objectionable harmonics which lie adjacent and below the output frequency need be corrected for by the length '74. The remaining objectional harmonics may be corrected for by selecting a length 78 from the iris 20 to the center of the varactor 44 such as will refleet a voltage minimum at the varactor 44 for the remaining undesired harmonics. Thereby maximum power is selectively transmitted at the desired harmonic to which the cavity l2 is tuned. The intensity of power transmitted from the varactor 44 to the cavity 12 may be varied by providing a tuning probe 80 which may be in the form of the body of a screw in the output side of the toll ticket 34 and preferably located and integral number of one eighth wave lengths from the center of the varactor 44.

To eliminate the problem of remaining harmonics from being propagated throughout the length 78 from the varactor 44 to the iris 20, the length 78 is selected such as to be an integral number of wave lengths of the remaining undesired harmonics.

The effect of the tuning probe 30 for diminishing power to the cavity 12 lies in its ability to detune the circuit in a manner to allow the propagation of harmonics parasitic to the power carried in the desired output frequency signal, thereby to control effective output frequency power.

This invention is not limited to the particular details of construction and operation described as equivalents will suggest themselves to those skilled in the art.

What is claimed is:

ll. In a microwave frequency multiplier, the combination of a cavity resonant at a selected microwave frequency, a source, of electrical energy at an integral subharmonic of the resonant frequency of the cavity, a substantially lossless and'predominantly reactive element having a non linear response characteristic positioned outside of and in remote relation to said cavity, means coupled to the electrical energy source and element for applying said integral subharmonic energy to the element and reflecting a voltage minimum at the element for undesired harmonics, and means coupled to the element and cavity for applying the selected frequency emanations of the element to the cavity with capacity for reflecting a voltage minimum at the element for undesired harmonics.

2. In a microwave frequency multiplier, the combination of a waveguide, a pair or irises spaced from each other in the waveguide to provide a cavity resonant at a desired output frequency, a substantially lossless and predominantly reactive element having a non linear response characteristic positioned outside of and in remote relation to said cavity, means coupled to the element for applying to the element electrical excitation having a frequency which is an integral subharmonic of the resonant frequency of the cavity, and reflecting a voltage minimum at the element for undesired harmonics, and means coupled to the element and cavity for applying the selected frequency emanations of the element to the cavity with capacity for reflecting a voltage minimum at the element for undesired harmonics.

3. In an apparatus for producing microwave frequency power the combination of a varactor, means coupled to the varactor for exciting the varactor with electrical energy having a frequency which is an integral subharmonic of the microwave frequency and reflecting a voltage minimum at the varactor for undesired harmonics, a tuned cavity separate and remote from the varactor and tuned to the microwave frequency and filter means coupled to the cavity and varactor with capacity for reflecting a voltage minimum at the varactor for undesired harmonics and suppressing substantially all power from the varactor to the cavity except the power at the desired microwave frequency.

4. In combination, a varactor, a rectangular waveguide housing the varactor and having its electric field plane dimensioned for substantially matching the impedance of the varactor for maximum power output of the varactor, a tuned cavity separate and remote from the varactor and tuned to the selected microwave frequency and having an input and output structures, filter means coupled to the cavity input structure and housing waveguide with capacity for reflecting a voltage minimum at the varactor for undesired harmonics for suppressing emanations from the varactor except those at the selected microwave frequency to the cavity and means for applying to the varactor an electric signal which is an integral subharmonic of the selected frequency.

S. In an apparatus for producing microwave energy at a preselected frequency, the combination of a source of electrical excitation at a frequency which is an integral sub-harmonic of the selected frequency, a substantially lossless and predominantly reactive element having a non linear reactance characteristic to excitation voltage, a rectangular waveguide housing the reactance element and having a depth cross sectional dimension parallel to the direction of the electric field in the TE mode and a width cross sectional dimension perpendicular to the direction of said electric field, the width dimension being such that the TE cutoff frequency of the waveguide housing the reactance element lies above the harmonic of the input frequency which is nearest and below the selected frequency and the depth dimension is proportioned to substantially match the impedance of the reactance element, irises in the waveguide defining a cavity separate and remote from said reactance element and resonant at the selected microwave frequency, a tapered waveguide transition coupling the reactance element to the cavity and effecting an electrical length for reflecting a voltage minimum at the element for undesired harmonics, and means coupling the source of electrical excitation and reactance element for applying the electrical excitation from the source to the reactance element with capacity for reflecting a voltage minimum at the element for undesired harmonics.

6. In an apparatus for producing microwave energy at a preselected frequency, the combination of a source of electrical excitation at a frequency which is an integral sub-harmonic of the selected frequency, a substantially lossless and predominantly reactive element having a non linear reactance characteristic to excitation voltage, a rectangular waveguide housing reactance element and having a depth cross sectional dimension parallel to the direction of the electrical eld in the TE. mode and a Width cross sectionl dimension perpendicular to the direction of said electric eld, the width dimension being such that the cutoif frequency of the waveguide Ihousing the reactance element lies above the harmonic of the input frequency which is nearest and below the selected frequency and the depth dimension is proportioned to substantially match the impedance of the reactance element, a cavity separate and remote from said reactance element and tuned to the selected microwave frequency and having an input and output structures, means coupling the reactance element to the cavity input structure for applying the selected frequency emanations of the reactance element to the cavity and reflecting a voltage minimum at the element for undesired harmonics, and means coupling the electrical excitation source and reactance element for applying the electrical excitation from the source to the reactance element and reflecting a voltage minimum at the element for undesired harmonics.

7. An apparatus as in claim 6 wherein the cavity is an electrical distance from the reactance element such that a voltage minimum is reected at the element for the nearest higher undesired harmonic above the selected frequency.

8. An apparatus as in claim 6 wherein the cavity is an electrical distance from the reactance element such that a voltage minimum is reected at the element for the nearest higher undesired harmonic above the selected frequency and a backshort is provided at a position opposite the cavity a distance from the reactance element such as to reflect a voltage minimum across the reactance element for undesired harmonics in the vicinity of the selected frequency above the cutoff frequency of the waveguide.

9. An apparatus as in claim 6 wherein the means coupling the electrical excitation source and reactance element includes a low-pass filter and a coaxial cable coupling the filter and reactance element and having an electrical length such that a voltage minimum is rellectcd from the low-pass lter across the coaxial cable at the reactance element for all harmonics of the input frequency lying below the selected frequency.

10. An apparatus as in claim 6 wherein the reactance element is a varactor, the cavity is an electrical distance from the varactor such that a voltage minimum is reilected at the varactor for the nearest higher undesired harmonic above the selected frequency, a backshort Ais provided at a position opposite the cavity a distance from the varactor for undesired harmonics in the vicinity of the selected frequency above the cutoff frequency of the waveguide, and the means coupling the electrical excitation source and varactor include a low-pass filter and a coaxial cable coupling the filter and varactor, the cable having an electrical length such that a voltage minimum is reflected from the low-pass filter across the coaxial cable at the varactor for all harmonics of the input frequency lying below the selected frequency.

References Cited by the Examiner UNITED STATES PATENTS 2,408,420 10/1946 Ginzton 321-60 2,460,109 1/ 1949 Southworth 321-60 2,817,760 12/1957 Dobbertin 321-69 2,969,497 1/1961 Zen-Iti Kiyasu et al. 321-69 2,970,275 l/1961 Kurzrok 330-4.9 2,982,922 5/1961 Wilson 321--69 3,060,364 10/1962 Holcomb 321-69 3,067,394 12/1962 Zimmerman et al 33.3-17 3,111,629 11/1963 Harris 330-4.9 3,114,881 12/ 1963 Uenohara 330-4.9

LLYOD MCCOLLUM, Primary Examiner.

SAMUEL BERNSTEIN, ROBERT L. SIMS, Examiners. 

6. IN A APPARATUS FOR PRODUCING MICROWAVE ENERGY AT A PRESELECTED FREQUENCY, THE COMBINATION OF A SOURCE OF ELECTRICAL EXCITATION AT A FREQUENCY WHICH IS AN INTEGRAL SUB-HARMONIC OF THE SELECTED FREQUENCY, A SUBSTANTIALLY LOSSLESS AND PREDOMINANTLY REACTIVE ELEMENT HAVING A NON LINEAR REACTANCE CHARACTERISTIC TO EXCITATION VOLTGE, A RECTANGULAR WAVEGUID HOUSING REACTANCE ELEMENT AND HAVING A DEPTH CROSS SECTIONAL DIMENSION PARALLEL TO THE DIRECTION OF THE ELECTRICAL FIELD IN THE TE. MODE AND A WIDTH CROSS SECTIONAL DIMENSION PERPENDICULAR TO THE DIRETION OF SAID ELECTRIC FIELD, THE WIDTH DIMENSION BEING SUCH THAT THE CUTOFF FREQUENCY OF THE WAVEGUIDE HOUSING THE REACTANCE ELEMENT LIES ABOVE THE HARMONIC OF THE INPUT FREQUENCY WHICH IS NEAREST AND BELOW THE SELECTED FREQUENCY AND THE DEPTH DIMENSION IS PROPORTIONED TO SUBSTANTIALLY MATCH THE IMPEDANCE OF THE REACTANCE ELEMENT, A CAVITGY SEPARATE AND REMOTE FROM SAID REACTANCE ELEMENT AND TUNED TO THE SELECTED MICROWAVE FREQUNECY AND HAVING AN INPUT AND OUTPUT STRUCTURES, MEANS COUPLING THE REACTANCE ELEMENT TO THE CAVITY INPUT STRUCTURE FOR APPLYING THE SELECTED FREQUENCY EMANATIONS OF THE REACTANCE ELEMENT TO THE CAVITY AND REFLECTING A VOLTAGE MINIMUM AT THE ELEMENT FOR UNDESIRED HARMONICS, AND MEANS COUPLING THE ELECTRICAL EXCITATION SOURCE AND REACTANCE ELEMENT FOR APPLYING THE ELECTRICAL EXCITATION FROM THE SOURCE TO THE REACTANCE ELEMENT AND REFLECTING A VOLTAGE MINIMUM AT THE ELEMENT OF UNDESIRED HARMONICS. 